Drive shapekey based on activating a pose?
You could try driving the shapekey and an action constraint from the same property or controller. Or control the action constraint's influence directly and have the influence drive the shapekey. This will require explicitly loading the pose though, you won't activate anything just freely posing the rig close to a given pose.

It is also possible to add a set of marker bones indicating the pose and then create drivers using 'Rotation Difference' variables between the marker bones an the actual bones and make it so the dirver is fully activated when the rotational difference is equal to zero. This may be vary complex to setup and tune for complex poses though, but does give the advantage of allowing free posing of the rig.